1. Field of the Invention
In many applications it is desirable to receive on a first frequency and transmit on a second frequency or to receive on two different frequencies or transmit on two different frequencies. In many instances, because of size limitations, cost or complexity, it is desirable to use the same antenna for both frequencies.
Specifically, in transducers for automatic positioning systems, radio object locating systems and in certain remotely controlled systems it is desirable to receive a signal at a first frequency and transmit the same or an answering signal at a second frequency. For example, the system may receive a base frequency and transmit a second harmonic of the base frequency.
2. Description of the Prior Art
In prior art dual frequency antennas, such as that disclosed in U.S. Pat. No. 2,479,227, entitled "Dual Frequency Antenna", issued on Aug. 16, 1949, to E. N. Gilbert, the antennna is extremely large and complicated.
A second type of dual frequency antenna is disclosed in U.S. Pat. No. 3,691,563, entitled "Dual Band Stripline Antenna", issued to Phillip L. Shelton on Sept. 12, 1972. This antenna is small compared to the above described structure but is relatively complicated to construct, compared to the present device.
In U.S. Pat. No. 3,343,089, entitled "Quarterwave Low Profile Antenna Tuned to Halfwave Resonance by Stub; also Including a Transistor Driving Stage", issued to E. R. Murphy, et al, on Sept. 19, 1967, and assigned to the same assignee as the present invention, a transmission line antenna is disclosed. However, this antenna is not a dual frequency antenna.